tee [OPTION]... [FILE]...

Copy standard input to each FILE, and also to standard output.

       -a, --append
              append to the given FILEs, do not overwrite

#define _GNU_SOURCE         /* See feature_test_macros(7) */
       #include <fcntl.h>

       ssize_t tee(int fd_in, int fd_out, size_t len, unsigned int flags);

tee()  duplicates  up to len bytes of data from the pipe referred to by
       the file  descriptor  fd_in  to  the  pipe  referred  to  by  the  file
       descriptor  fd_out.   It  does  not consume the data that is duplicated
       from fd_in;  therefore,  that  data  can  be  copied  by  a  subsequent
       splice(2).